1. Field of the Invention
The present invention relates in general to a method for the simultaneous deposition of chromium and silicon to form a diffusion coating in metals, and in particular to an improved method for the codeposition of chromium and silicon to form a diffusion coating in steel using dual activator salts.
2. Description of the Related Art
It is known in the field of coating metals or alloys to use a pack cementation process. Basically, a pack cementation process is a modified chemical vapor deposition process which consists of heating a closed or vented pack to an elevated temperature for a specific amount of time during which a diffusional coating is produced on a metal. The closed or vented cementation pack is protected from oxidation by an inert or reducing atmosphere. The cementation pack consists of the metal or alloy member or substrate which is to be coated, surrounded by the elements to be deposited (usually in the form of metal or masteralloy powders), a halide activator salt, and a powder filler. An inert gas, such as argon, or else hydrogen is used to surround the pack. Once the pack is heated to a sufficiently elevated temperature, the activator salt reacts with the metal or masteralloy powder to form metal halide vapors. The metal halide vapors diffuse to the substrate or metal surface through the gas phase of the porous pack. At the substrate surface, a reaction step results in deposition of the desired element and the formation by solid state diffusion of a protective coating at the metal surface. The surface reaction may be somewhat complex, involving adsorption, dissociation, and/or surface diffusion of the molecular species.
In the past, most commercial cementation coating processes have involved the deposition of single elements such as aluminum, chromium or silicon. U.S. Pat. No. 5,364,659 to Rapp et al. describes a method for the codeposition of chromium and silicon diffusion coatings on a steel using a pack cementation process. The specific dual activators of NaF and NaCl were employed to codeposit chromium and silicon to achieve a desired composition (i.e., 25-30 wt. % Cr and 3-4 wt. % Si) in a process that requires an exact control of the fluxes of Cr and Si from the pack to the workpiece during the coating process. This process required the selection of a Cr--Si masteralloy with the desired component activities and a silica filler. The use of the proper ratio of salts as dual activators in combination with the use of a reactive silica filler serves to adjust the partial pressures of chromium chloride and silicon fluoride to set the fluxes of the chromium and silicon into the metal in the right proportion. The foregoing process specifies the use of a Cr--Si masteralloy powder which is expensive and probably cannot be recycled/upgraded. In addition, while the process was successful for relatively low carbon metals, e.g. 2.25 Cr-1Mo-0.15C, an external carbide was formed for higher carbon steels which disrupted the inward diffusion of chromium and silicon. During the process, the substrate was decarburized, thus reducing the strength of the steel. Additionally, the foregoing process did not have any provision for the introduction into the coating of a small concentration (&lt;1%) of a reactive element such as cerium, which is known to provide a number of advantages in scale adherence and reduced sealing kinetics. Likewise, the foregoing process did not have any provision for the introduction of a small vanadium content (.gtoreq.0.5% V) in the coating. Such a vanadium addition is known to improve the aqueous corrosion resistance.
Accordingly, there is a need for an improved chromium and silicon diffusion coating process which addresses the problem of a blocking chromium carbide layer formed at the surface and which provides a means for the introduction into the coating of a small concentration of reactive element such as cerium, or of vanadium. Preferably, the improved process would use a mixture of powders that is less expensive and incorporate a processing schedule that would not affect the strength of the metal. It is desirable for the improved method to form a coating with a high alloy content on a medium carbon steel or a high strength low alloy steel which could also offer corrosion resistance in oxidizing and corrosive environments at elevated temperatures. Likewise, such coatings offer exceptional resistance to corrosion in aggressive aqueous solutions.